Lithium ion batteries are of great technological interest for application in a variety of contexts, ranging from cell phones, laptop computers, and electric vehicles, to use in energy backup systems. Their broadest usage at present is in portable electronic devices.
Lithium ion batteries consist of two lithium intercalation electrodes (an anode and a cathode) and an ionically-conducting electrolyte. A suitable cathode material should have a high potential versus lithium, reversibly insert lithium ions without losing structural integrity, and not induce reactions with the electrolyte. Candidate materials for lithium ion battery cathodes are typically lithiated transition metal oxides. The most commercially used cathode materials are described by the formulae LiCoO2 and LiCoxNi1−xO2(x<0.3), and deliver moderate capacity and good cyclability. However, the cost and safety concerns (notably, the tendency toward instability at low lithium contents) associated with these compounds make them less than ideal. Thus, much research has taken place to find alternatives to LiCoxNi1−xO2 cathode materials.
One widely studied species is LiMn2O4 and its derivatives, also known as spinel. Unfortunately, these materials experience Mn2+ dissolution into the electrolyte, producing rapid capacity loss on cycling or storage above room temperature. Substitution of a portion of the Mn by other elements (Li, Mg, Zn, or Al, for example) creates compositions with greater stability, but the initial reversible capacity is reduced substantially.
Another alternative which has been suggested, and which possesses the same layered structure as LiCoxNi1−xO2, is LiMnO2 and its derivatives, in which the Mn is partially replaced with a stabilizing cation. U.S. Pat. No. 6,214,493 to Bruce et al. discloses in this regard that laminar LiyMnO2, a material inherently unstable when delithiated, is much less susceptible to structural degradation when another metal replaces a minority portion of the Mn. The stabilizing element may be any metal, but is most typically a transition metal in its +3 oxidation state, including Co, Al, and Fe. A further refinement taught by Bruce is the addition of ‘excess’ lithium (i.e., y>1) to strengthen the lattice.
Patent Cooperation Treaty Application No. PCT/US02/24684, published as WO 03/015198A2 (Dahn et al), describes materials with the formula Liy[M1−bMnb]O2, where 0≦y<1,0<b<1 and M is Ni, Co or Fe, and wherein the layered O3 crystal structure is maintained (identical to Li[Li1/3Mn2/3]O2). Materials are described as attaining a capacity of 130 mAh/g when charged t 4.3V, similar to spinel and LiCoxNi1−xO2, while charging above 4.5V leads to capacities exceeding 225 mAh/g.
Li2MnO3 (Li[L1/3Mn2/3]O2) and its structural derivatives have also been investigated. Thus, Numata et al. in Solid State Ionics, no. 117, pp. 257-263 (1997) describe a stable, battery-active solid solution of LiCoO2 and Li2MnO3 having the layered structure of the Co compound. Still other references describe layered lithium manganese nickel oxide cathode materials derived from Li2MnO3 and which have the formulae Li[Li(1-2x)/3Mn(2−)/3Nix]O2, see, for example, Lu et al., Electrochemical and Solid State Letters, vol. 4, no. 11, pp A191-A1094 (2001), Lu et al., Journal of the Electrochemical Society, vol. 149, no. 6 pp A778-A791 (2002), and Shin et al., Journal of Power Sources 112, pp 634-638 (2002).
More recently, EP 1 189 296A2 relates the finding by Paulsen et al. that replacing a fraction of the Ni2+ with Co3+ produces intercalation compounds with greater rate capability than the parent LMN family or LiMO2 end members. Both the MnNi and CoMnNi series have significant capacities above 4.2V, but this is a voltage region incompatible with organic electrolyte solvents (including polymers). At such elevated potentials, current flow is forced by electrochemically decomposing solvent molecules into ionic species, most notably producing protons, which may dissolve some of the transition metal out of the cathode. The dissolved species and molecular fragments from the electrolyte subsequently accumulate on the electrodes (typically the anode) and interfere with lithium ion transport, reducing cell capacity.